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Abstract

We present an experimental study on the formation and dissociation characteristics of carbon dioxide (CO2) gas hydrates using Raman spectroscopy. The CO2 hydrates were formed from sodium-chloride/water solutions with salinities of 0-10 wt-%, which were pressurized with liquid CO2 in a stirred vessel at 6 MPa and a subcooling of 9.5 K. The formation of the CO2 hydrate resulted in a hydrate gel where the solid hydrate can be considered as the continuous phase that includes small amounts of a dispersed liquid water-rich phase that has not been converted to hydrate. During the hydrate formation process we quantified the fraction of solid hydrate xH and the fraction of the dispersed liquid water-rich phase xL from the signature of the hydroxyl (OH)-stretching vibration of the hydrate gel. We found that the fraction of hydrate xHcontained in the hydrate gel linearly depends on the salinity of the initial liquid water-rich phase. In addition the ratio of CO2 and water was analyzed in the liquid water-rich phase before hydrate formation, in the hydrate gel during growth and dissociation and after its complete dissociation again in the liquid water-rich phase. We observed a supersaturation of CO2 in the water-rich phase after complete dissociation of the hydrate gel and were able to show that the excess CO2 exists as dispersed micro- or nanoscale liquid droplets in the liquid water-rich phase. These residual nano- and micro-droplets could be a possible explanation for the so-called memory effect.